1. Field of the Invention
The present invention relates to substantially non-yellowing radiation-curable protective coating compositions, and in particular, compositions which are tailored for protecting and/or bonding optical fibers within an optical fiber ribbon or cable structure.
2. Description of Related Art
Glass optical fibers have become a medium of choice for transmitting information in the modern telecommunications era. Immediately after their manufacture, the fibers are coated with relatively soft inner primary coatings, which directly contact the underlying glass optical fibers. They are then coated with harder outer primary coatings which overlay the inner primary coatings (outer primary coatings are also called secondary coatings). This dual coating structure both maximizes fiber transmission efficiency and preserves the desirable characteristics of freshly-prepared, pristine glass fiber. To maximize telecommunication efficiency, multiple strands of coated optical fibers are further encased in ribbons and cables. Tertiary coatings and jacketing, bundling, and matrix materials further identify and protect the glass fiber and bond bundles of fibers in ribbon and cable structures. Radiation-curable materials are particularly useful in this art because they allow for fast production of coated fiber and ribbons. These practices are conventional in the optical fiber art.
In one aspect of this technology, colorants or inks can be used to help distinguish one strand of optical fiber from another simply by color. Color is important when, for example, repair and/or splicing of the optical fiber is needed. However, aging can make such color differentiation difficult if color changes substantially over time. Substantially colorless materials must remain colorless, and the color of colored materials must not change despite environmental stress. Environmental stresses include, for example, light, oxidation, temperature, humidity, water, acid, base, chemicals, and solvents. Severe short term aging studies on coated fiber help predict the long-term reliability of the fiber. For substantially colorless materials, yellowing is the most common form of discoloration. Discoloration and yellowing is conventionally measured in terms of the delta E parameter.
Prior art references recognize the yellowing problem. See, for example, U.S. Pat. Nos. 5,146,531 to Shustack and 4,962,992 to Chapin et al. See also, Lightguide Digest, 1992, No. 1, pgs. 2-5. These references disclose coated optical fibers and radiation-curable urethane acrylate coating compositions which allegedly demonstrate improved non-yellowing behavior. However, the Chapin patent discloses that the outer primary coating should have a sufficiently low glass transition temperature (Tg&lt;60.degree. C.) to avoid delamination of the coating system from the optical fiber and provide suitable resistance to microbending. Consistent with this, the Shustack patent discloses a Tg value of only 50.degree. C. Hence, there is no motivation in these references to prepare materials with Tg higher than 50.degree. C.
A related disclosure for allegedly non_yellowing optical fiber primary and secondary coating compositions is U.S. Pat. No. 5,352,712 to Shustack. According to this patent, the outer primary coating again should have a glass transition temperature of about 50.degree. C., so again, there is no teaching or suggestion to elevate Tg above a value of about 50.degree. C. Also, U.S. Pat. No. 5,527,835 to Shustack discloses that coatings are to be non-yellowing but does not suggest outer primary coatings having a relatively high Tg.
Furthermore, U.S. Pat. No. 5,093,386 to Bishop et al. discloses polyether-based compositions useful as a secondary coatings or bundling materials. However, these compositions require use of a polyurethane having a tricyclodecane structure in the backbone. This structure can be undesirable because it imparts high viscosity.
Acrylated epoxy types of optical fiber coatings are generally viewed as prone to yellowing, as discussed in, for example, the aforementioned U.S. Pat. No. 5,146,531. In addition, it is generally understood that coatings photodegrade when based on acrylated epoxy derivatives of bisphenol A. See the publication, "Radiation Curable Coatings; A Technology for the 1980's" by G. Pasternack in "The Proceedings of the 1980 Paper Synthetic Conference," Cincinnati, Ohio, September 1980. In particular, yellowing has been attributed to the aromatic character of acrylated epoxies. Hence, it would be surprising if aromatic-based coatings are non-yellowing.
Aspects of thermooxidative and hydrolytic degradation, including yellowing and delta E measurements, are discussed in T. Bishop et. al. International Wire & Cable Symposium Proceedings, 1992, pgs. 442-446. However, there is no suggestion to prepare compositions having relatively high Tg which also have the substantially non-yellowing character disclosed herein.
Hence, discoloration in general and yellowing in particular is a problem in the optical fiber coating industry. Moreover, substantial non-yellowing compositions are difficult to achieve which also exhibit other required properties. Modern optical fiber technology depends on, and urgently demands, better substantially non-yellowing optical fiber protective materials with a better balance of properties.
Objects of the present invention include providing radiation-curable compositions which, when cured, do not substantially yellow (i.e., low delta E) and yet have other desirable properties. In particular, these materials should yellow less than the allegedly non-yellowing coatings disclosed in, for example, the aforementioned U.S. Pat. Nos. 5,146,531 and 5,352,712 to Shustack. These and other objects have been achieved.